The role of parliament has been regarded as limited in foreign and defence affairs in Britain. The immediate post-war situation of 1945–6 was characterised by numerous changes and problems, one of them being the question of atomic energy. By using this issue as a case study, this article considers how, despite various limitations, parliament contributed to defining the question as a foreign affairs matter and attempted to parliamentarise it. Emphasis is given to the role of information, the ‘everyday’ interaction and dynamics between the executive and parliament, which, combined with ‘parliamentary momentum’ and other factors, gave parliament the possibility to challenge government domination, which was a predominating aspect of the political culture.